Adiabatic sweep cross-polarization magic-angle-spinning NMR of half-integer quadrupolar spins

Abstract

The use of frequency-swept radiofrequency (rf) pulses for enhancing signals in the magic-angle spinning (MAS) spectra of half-integer quadrupolar nuclides was explored. The broadband adiabatic inversion cross-polarization magic-angle spinning (BRAIN-CPMAS) method, involving an adiabatic inversion pulse on the S-channel and a simultaneous rectangular spin-lock pulse on the I-channel (H), was applied to I(1/2)→S(3/2) systems. Optimal BRAIN-CPMAS matching conditions were found to involve low rf pulse strengths for both the I- and S-spin channels. At these low and easily attainable rf field strengths, level-crossing events among the energy levels |3/2〉,|1/2〉,|-1/2〉,|-3/2〉 that are known to complicate the CPMAS of quadrupolar nuclei, are mostly avoided. Zero- and double-quantum polarization transfer modes, akin to those we have observed for I(1/2)→S(1/2) polarization transfers, were evidenced by these analyses even in the presence of the quadrupolar interaction. H-Na and H-B BRAIN-CPMAS conditions were experimentally explored on model compounds by optimizing the width of the adiabatic sweep, as well as the rf pulse powers of the H andNa/B channels, for different MAS rates. The experimental data obtained on model compounds containing spin-3/2 nuclides, matched well predictions from numerical simulations and from an average Hamiltonian theory model. Extensions to half-integer spin nuclides with higher spins and potential applications of this BRAIN-CPMAS approach are discussed.

Practical Aspects of Recording Ultra-Wideline NMR Patterns under Magic-Angle Spinning Conditions

“…Bodenhausen, Amoureux, and co-workers reported a theoretical treatment describing the use of the DANTE pulse sequence for the acquisition of UWNMR spectra (examples for 19 F and 14 N were provided) . The use of the dipolar-HMQC (D-HMQC) pulse sequence (and several variants) for the acquisition of indirectly detected UWNMR spectra of both spin- 1 / 2 and quadrupolar nuclei have been explored by Amoureux et al , and Rossini et al − Finally, Wi et al have been carrying out studies featuring the use of BRAIN pulses for BCP to both spin- 1 / 2 and quadrupolar nuclei (applications for UWNMR have not yet been reported). − To date, there have been no reports of a reliable method for obtaining directly detected 1D UWNMR spectra of spin- 1 / 2 or quadrupolar nuclei using direct excitation (DE) or CP methods under MAS conditions, nor have there been any reports of implementations of CPMG-style sequences or frequency-swept pulses like WURST pulses for these purposes.…”

Quadrupolar Isotope-Correlation Spectroscopy in Solid-State NMR

“…After suitable tuning, varying the flip angle of 85 Rb clearly reflected in the phased CPMG signal of the 87 Rb spectrum, verifying the direct correlation of both isotopes (Figure a). Notice that complexities associated with CP between half-integer quadrupoles undergoing MAS ,− will be absent under QUICSY’s static conditions, which are compatible with conventional CP transfer protocols. Still, bandwidth and relaxation limitations may arise, particularly given the short T 1 s and relatively low B 1 s for low-γ nuclei such as 85 Rb.…”

Broadband adiabatic inversion cross‐polarization to integer‐spin nuclei with application to deuterium NMR

“…The BRAIN-CP pulse sequence has previously been implemented for 1 H- 14 N experiments [29] ; however, a theoretical description of the spin dynamics and HH matching conditions was not provided in the same detail as the static spin-1/2 case, [28] or for quadrupolar systems under magic-angle spinning (MAS). [32][33][34] We present an analytical derivation of the HH matching conditions during irradiation with an FS pulse, for CP from a spin-1/2 nucleus to an integer-spin nucleus, under static conditions. The spin dynamics and BRAIN-CP spin polarization transfer mechanisms are monitored using numerical simulations in conjunction with the analytical expressions.…”